A. Field of the Invention
The invention relates generally to an apparatus and a method, for the effective application of a heat shrinkable film or label, to a container having a right-circular, cylindrical sidewall section, and at least one inwardly directed end section. More specifically, the invention provides a novel solution to the problem of forming an effective adhesive seam between overlapping ends in the free-standing portion of a heat shrinkable film. After formation of the overlap seam, heat is applied to the label, shrinking the free-standing portion over the container's contoured end section to provide a tight and attractive container wrap.
Certain advantages and difficulties associated with the use and application of heat shrinkable film on containers are discussed in U.S. Pat. No. 4,923,557, issued to Dickey, and assigned to the predecessor in interest of the assignee herein. The entirety of U.S. Pat. No. 4,923,557 is specifically incorporated by reference into the instant application. The '557 Patent teaches that compressive forces can advantageously be applied outwardly, against the inner wall of overlapping but unsupported ends of a solvent reactive film or label, to form an effective solvent induced bond or seal therebetween.
Having an extremely low viscosity, the solvent bonding agent used to practice the invention of the '557 Patent spreads rapidly by capillary action between the leading and trailing ends of a label, overlapped during the labeling process. It is critical, however, that these label ends are maintained in contingent relation, during the moment of initial overlap, before the solvent evaporates. The force required to make a positive, solvent induced seal is not great. As long as sufficient pressure is applied at the time of overlap to keep the ends physically together, an effective bond will be formed between the two ends.
Nevertheless, economic considerations may make it desirable to label a container with a film material which is heat shrinkable, but not solvent reactive. Heat shrinkable polypropylene, for example, is less expensive than the solvent-sensitive polyvinylchloride label material. Polyethylene also shows promise as another economical heat shrinkable material, which is not solvent reactive. In addition to cost factors, health or environmental concerns may dictate the absence of solvent in any aspect of particular labeling processes.
In these instances, an adhesive or glue must be applied first on the leading end of the heat shrinkable label to adhere the label to the container, and then on the trailing end of the label, to form an overlapping seam with the leading end. During this application process, the label is usually transported by a rotary vacuum drum or a vacuum conveyor belt, past a glue station. There, a glue applicator element is typically brought into direct contact only with selective portions of the label. Alternatively, a gravure element has been employed to print adhesive only upon the desired areas of the label. The prior art also teaches the application of glue upon a label from an ejection nozzle, spaced from the vacuum drum.
A widely used, conventional adhesive in the labeling art is hot melt adhesive, manufactured from a resinous material, characteristically solidified at room temperature. Hot melt adhesive is applied in a fluid, but significantly viscous condition, generally both to leading and trailing ends of a label segment, before wrap-applying the segment around a container. Other adhesives, such as water-based paste or glue, have similarly been used either alone or in combination with hot melt adhesive, to adhere full wrap or partial wrap labels to containers.
Certain characteristics of these adhesives and glues, particularly those of hot melt adhesive, make their use more difficult in heat shrink label applications: (1) owing to its high viscosity, hot melt adhesive does not readily "wet" the label material and spread under overlapping label ends, as solvent does; (2) a coating or layer of hot melt on a label cools from its exposed surface to its interior portion, with the consequence that the exposed portion of the adhesive initially making contact between label ends, is cooler and less tacky than the interior portion; (3) hot melt applied to a relatively thin, heat shrinkable film causes the film to curl slightly in the immediate area of application, making positive, full contact between the label ends problematical. The known prior art does not address these difficulties in the same manner as that contemplated by the invention disclosed and claimed herein.
B. Description of the Prior Art
U. S. Pat. No. 4,844,760, issued to Dickey, discloses the use of ejection-applied hot melt adhesive and wipe-applied solvent, respectively, upon leading and trailing ends of a solvent reactive label, prior to applying the label to a container.
U.S. Pat. No. 4,923,557, issued to Dickey, teaches the use of compressive force against the inner wall of an unsupported solvent formed label seam, while the outer wall of the film seam is backed by a vacuum drum.
U.S. Pat. No. 4,406,721, granted to Hoffmann, illustrates an extensible-retractable tongue mechanism, to facilitate the formation of an adhesive seam in the label ends compressed between the tongue and a vacuum drum.
U.S. Pat. No. 4,872,931, issued to Mitchell, shows a container gripping chuck mechanism, having an integral lip-extension. A vacuum drum presses against the label seam, backed by the chuck extension, to form an adhesive bond.
U.S. Pat. No. 4,544,431, granted to King, discusses the use of hot melt adhesives and solvent reactive adhesives, in which a timed air blast is used to drive the trailing end of the label against a column coated with glue or solvent.